FIG. 1 shows a schematic of a traditional connection of a loop filter to a LC tank VCO in phase locked loop circuits. In the schematic of FIG. 1, the voltage v is applied to one end of a set of capacitors in a filter element whose value determines, along with the value of the inductance, the oscillation frequency of the VCO.
More specifically as shown in FIG. 1, in the conventional VCO architecture, a charge pump feeds into an RCC filter. The charge pump pumps current pulses in or draws current pulses out based on signals INC and DEC which are the phase detector outputs. In the conventional VCO architecture, the RCC filter has two main components, a resistor R and a capacitor C1. The output of R and C1 is VCNTL which is used to control the capacitance value to change the frequency. A second capacitor C2 is used to filter out high frequency variations. Since C2 is much lower than C1 the majority of the charge pump current pulse flows in the R-C1 path.
In the circuitry of FIG. 1, the proportional control is the resistor R and the integral control is the capacitor C1. The proportional control causes a different change in frequency based on whether the VCO is working at a low or a high frequency. As such, as the VCNTL is used to control the capacitance value to change the frequency, the gain of the VCO will change in frequency at a change in voltage at the VCTNL. This being the case, the gain is higher at a higher frequency and lower at a lower frequency.
More specifically, the current pulse i flowing in the resistor R creates a voltage pulse which forms the proportional control (PC), and the current pulse flowing in C1 forms an integral control (IC) since the voltage change is the integral of the current pulse. The voltage v thus created at the filter node VCNTL is described as:
                    υ        =                              i            ×            R                    +                                    1                              C                ⁢                                                                  ⁢                1                                      ×                          ∫                              i                ⁢                                  ⅆ                  t                                                                                        (        1        )            
The proportional control signal corresponds to the first part of equation (1). In this case, it is seen that the PC portion of the control voltage changes immediately as the current i into the filter changes. Also, the IC portion is a slower change as it is an integration of the equation.
Thus, in the configuration of FIG. 1 the current pulse increases the voltage v at node VCNTL which is reflected in the frequency increase. The effect of the proportional control is to thus immediately increase the frequency since the voltage v increases immediately as shown in equation (1). The frequency change caused by the voltage v is:Δf=K×v  (2)K is the gain of the VCO expressed in MHz/V, and is determined by the size of the variable capacitors connected to VCNTL. Also, the voltage v causes a change in the capacitance value which causes a frequency change. The parameter K captures both changes in one variable.
FIG. 2 shows an open loop gain GH for a conventional filter as the VCO gain changes. It is seen that the loop crossover frequency changes with gain. FIG. 3 shows a closed loop input-output gain as the VCO gain changes. It is seen that the −3 dB bandwidth changes with gain. As such, this makes it difficult to control the noise at the PLL output since it is this bandwidth that controls the amount of input noise that is fed through to the output.
Accordingly, there exists a need in the art to overcome the deficiencies and limitations described hereinabove.